Gametogenesis and Reproduction of Meloidogyne graminis and M. ottersoni (Nematoda: Heteroderidae)

Oogenesis and spermatogenesis of seven populations of Meloidogyne graminis and one population of M. ottersoni (formerly Hypsoperine spp.) were of the meiotic type. When males were abundant, reproduction was by amphirnixis. In most greenhouse cultures, however, males were rare and reproduction was by meiotic parthenogenesis. M. graminis and M. ottersoni are closely related to each other and to M. graminicola and M. naasi, but differ in some respect from other Meloidogyne species. It is suggested that these four species be treated together as a group of species, either in the genus Meloidogyne or in the genus Hypsoperine.     

Relative DNA Content and Chromosomal Relationships of some Meloidogyne, Heterodera, and Meloidodera spp. (Nematoda: Heteroderidae)

The relative DNA content of hypodermal nuclei of preparasitic, 2nd-stage larvae was determined cytophotometrically in 19 populations belonging to 13 species of Meloidogyne, Heterodera and Meloidodera. In Meloidogyne hapla, M. arenaria, M. incognita and M. javanica, total DNA content per nucleus is proportional to their chromosome number, indicating that chromosomal forms with high chromosome numbers are truly polyploid. M. graminicola, M. grarninis and M. ottersoni have a DNA content per chromosome significantly lower than that of the other Meloidogyne species. Within Heterodera, species with high chromosome numbers have proportionally higher DNA content, indicating again polyploidy. DNA content per chromosome in Meloidogyne is one third that of Heterodera and one haft that of Meloidodera floridensis. The karyotypic relationships of the three genera are still not clearly understood.     

Enzymatic Relationships and Evolution in the Genus Meloidogyne (Nematoda: Tylenchida)

Thirty populations of Meloidogyne of diverse geographic origin representing 10 nominal species and various reproductive, cytological, and physiological forms known to exist in the genus were examined to determine their enzymatic relationships. The 184 bands resolved in the study of 27 enzymes were considered as independent characters. Pair-wise comparisons of populations were performed in all possible combinations to estimate the enzymatic distances (ED) and coefficients of similarity (S). A phylogenetic tree was constructed. The apomictic species M. arenaria, M. microcephala, M. javanica, and M. incognita shared a common lineage. M. arenaria was highly polytypic, whereas conspecific populations of M. javanica and M. incognita were largely monomorphic. The mitotic and meiotic forms of M. hapla were very similar (S = 0.93), suggesting that the apomictic race B evolved only recently from the meiotic race A. The five remaining meiotic species (M. chitwoodi, M. graminicola, M. graminis, M. microtyla, and M. naasi - each represented by a single population) were not closely related to each other or to the mitotic species.     

Life Cycle,Pathogenicity, Histopathology,and Host Range of Race 5 of the Barley Root-Knot Nematode

The optimum temperature for development of race 5 of Meloidogyne naasi was 26 C. A life cycle was completed in 34 days. Growth of sorghum was suppressed when inoculated with M. naasi. Observations of M. naasi-infected sorghum roots demonstrated that roots were penetrated just behind the root cap; giant cells were generally initiated either in the procambial region or in very young phloem. When giant cells developed in the cortex, corresponding areas of the vascular system did not have an endodermis, pericycle, or phloem fibers. Nineteen plant species were tested for suitability as hosts for race 5 of M. naasi. Reproduction occurred on 11 of 12 monocotolydenous hosts and none of 7 dicotolydenous hosts. Reproduction often occurred without gall development.     

Die Zytologie der bisexuellen und parthenogenetischen Fortpflanzung von Heteropeza pygmaea Winnertz,einer Gallmücke mit pädogenetischer Vermehrung

Heteropeza pygmaea (syn. Oligarces paradoxus) can reproduce as larvae by paedogenesis or as imagines (Fig. 1). The eggs of imagines may develop after fertilization or parthenogenetically. The fertilized eggs give rise to female larvae, which develop into mother-larvae with female offspring (Weibchenmütter). Only a few of the larvae which hatch from unfertilized eggs become motherlarvae with female offspring; the others die. Spermatogenesis is aberrant, as it is in all gall midges studied to date. The primary spermatocyte contains 53 or 63 chromosomes. The meiotic divisions give rise to two sperms each of which contains only 7 chromosomes (Figs. 5–11). The eggs of the imago are composed of the oocyte and the nurse-cell chamber. In addition to the oocyte nucleus and the nurse-cell nuclei there are three other nuclei in the eggs (Figs. 15–17). They are called small nuclei (kleine Kerne). In prometaphase stages of the first cleavage division it could be seen that these nuclei contain about 10 chromosomes. Therefore it is assumed that these nuclei originate from the soma of the mother-larva. The chromosome number of the primary oocyte is approximately 66. The oocyte completes two meiotic divisions. The reduced egg nucleus contains approximately 33 chromosomes. The polar body-nuclei degenerate during the first cleavage divisions. The fertilized egg contains 2–3 sperms. The primary cleavage nucleus is formed by the egg nucleus and usually all of the sperm nuclei and the small nuclei (Figs. 21–29). The most frequent chromosome numbers in the primary cleavage nuclei are about 77 and 67. The first and the second cleavage divisions are normal. A first elimination occurs in the 3rd, 4th, and 5th cleavage division (Fig. 30). All except 6 chromosomes are eliminated from the future somatic nuclei. Following a second elimination (Figs. 33, 34), the future somatic nuclei contain 5 chromosomes. No elimination occurs in the divisions of the germ line nucleus. In eggs which develop parthenogenetically the primary cleavage nucleus is formed by the egg nucleus and 2–3 small nuclei. It's chromosome number is therefore about 53 or 63. After two eliminations, which are similar to the ones which occur in fertilized eggs, the soma contains 5 chromosomes. The somatic nuclei of male larvae which arrise by paedogenesis contain 5 chromosomes; while the somatic nuclei of female larvae of paedogenetic origin contain 10 chromosomes. It was therefore assumed earlier that sex was determined by haploidy or diploidy. But the above results show that larvae from fertilized as well as from unfertilized eggs of imagines have 5 chromosomes in the soma, but are females, and the female paedogenetic offspring of larvae from unfertilized eggs have either 5 or 10 chromosomes in their somatic cells. Therefore sex determination is not by haploidy-diploidy but by some other, unknown, mechanism. The cytological events associated with paedogenetic, bisexual, and parthenogenetic reproduction in Heteropeza pygmaea are compared (Fig. 37). The occurrence and meaning of the small nuclei which are found in the eggs of most gall midges are discussed. It has been shown here that these nuclei function to restore the chromosome number in fertilized eggs; it is suggested that they function similarity in certain other gall midges. Consideration of the mode of restoration of the germ-line chromosome number leads to the conclusion that in Heteropeza few, if any, of the chromosomes are limited to the germ-line, i.e. can never occur in somatic cells (p. 124).     

The cytology of paedogenesis in the gall midgeMycophila speyeri

In paedogenetically developing female eggs of the gall midgeMycophila speyeri only one equational meiotic division occurs. The primary cleavage nucleus contains 29 chromosomes. In the fourth cleavage division 23 chromosomes are eliminated from the future somatic nuclei while the primordial germ-line nucleus keeps the high chromosome number.—The paedogenetic development of male eggs begins with two meiotic divisions. The egg nucleus with 14 or 15 chromosomes fuses with two, sometimes only one, somatic nuclei (2n=6) of maternal origin (regulation). Thus the primary cleavage nucleus contains 26 or 27 chromosomes, sometimes only 20 or 21. Elimination in cleavage divisions V and VI leeds to somatic nuclei with 3 chromosomes while the primordial germ-line nucleus keeps the high chromosome number.—Differences between male and female eggs and the evolution of regulation in gall midges are discussed.     

Comparison of Reproduction by Meloidogyne graminicola and M. incognita on Trifolium Species

The reproductive potential of Meloidogyne graminicola was compared with that of M. incognita on Trifolium species in greenhouse studies. Twenty-five Trifolium plant introductions, cultivars, or populations representing 23 species were evaluated for nematode reproduction and root galling 45 days after inoculation with 3,000 eggs of M. graminicola or M. incognita. Root galling and egg production by the two root-knot nematode species was similar on most of the Trifolium species. In a separate study, the effect of initial population densities (Pi) of M. graminicola and M. incognita on the growth of white clover (T. repens) was determined. Reproductive and pathogenic capabilities of M. graminicola and M. incognita on Trifolium spp. were similar. Pi levels of both root-knot nematode species as low as 125 eggs per 10-cm-d pots severely galled white clover plants after 90 days. Meloidogyne graminicola has the potential to be a major pest of Trifolium species in the southeastern United States.     

Interaction of Meloidogyne naasi,Pratylenchus penetrans,and Tylenchorhynchus agri on Creeping Beentgrass

The pathogenicity and interactions of Meloidogyne naasi, Pratylenchus penetrans, and Tylenchorhynchus agri on ''Toronto C-15'' creeping bentgrass, Agrostis palustris, was studied in a long-term greenhouse experiment. Based on dry weights of roots and clippings, M. naasi alone and in all combinations with P. penetrans and T. agri was highly pathogenic to creeping bentgrass. P. penetrans and T. agri alone and in combination inhibited root growth but adversely affected top growth only when the two were co-inoculated. In combination, the effects of each species on top growth were additive, with M. naasi the dominant pathogen. Creeping bentgrass was an excellent host for M. naasi and T. agri, but a poor host for P. penetrans. T. agri inhibited population increase of M. naasi, indicating nematode-nematode competition, but neither T. agr/ nor P. penetrans was affected by any of the combinations.     

Oogenesis and Embryology of Two Plant-parasitic Nematodes,Pratylenchus penetrans and P. zeae

The process of oogenesis was studied in the bisexual species, Pratylenchus penetrans, and the monosexual species, P. zeae. The nucleus of the oocyte of P. penetrans underwent two divisions after sperm penetration. The chromosome number of P. penetrans observed at metaphase of the first maturation division was 2n = 10 and the reduced chromosome number observed at anaphase of the first maturation division was n = 5. Two polar bodies were found within the egg, indicating that this species reproduces by amphimixis. The nucleus o f the oocyte of P. zeae underwent one mitotic division and the chromosome number was 2n = 26. The presence of only 1 polar body indicates that this species reproduces by mitotic parthenogenesis. The development of the embryo was similar in P. penetrans and P. zeae. Unsegmented eggs were usually deposited by females. Following the 9-celled stage, the number of cells increased rapidly until a blastula was formed. Cell differentiation immediately followed, as evidenced by the formation of darker and larger inner endodermal cells and smaller ectodermal cells. Six to 7 days after egg deposition, the first stage larva was coiled three to f o u r times within the egg shell. During the first molt, the styler apex was formed first, then the larva moved frequently and vigorously and the styler was repeatedly thrust into the egg shell. Finally, the shell was broken and the second stage larva emerged. It took 10 days from the unsegmented egg to hatching at 23 C.     

Meiosis Drives Extraordinary Genome Plasticity in the Haploid Fungal Plant Pathogen Mycosphaerella graminicola

Meiosis in the haploid plant-pathogenic fungus Mycosphaerella graminicola results in eight ascospores due to a mitotic division following the two meiotic divisions. The transient diploid phase allows for recombination among homologous chromosomes. However, some chromosomes of M. graminicola lack homologs and do not pair during meiosis. Because these chromosomes are not present universally in the genome of the organism they can be considered to be dispensable. To analyze the meiotic transmission of unequal chromosome numbers, two segregating populations were generated by crossing genetically unrelated parent isolates originating from Algeria and The Netherlands that had pathogenicity towards durum or bread wheat, respectively. Detailed genetic analyses of these progenies using high-density mapping (1793 DArT, 258 AFLP and 25 SSR markers) and graphical genotyping revealed that M. graminicola has up to eight dispensable chromosomes, the highest number reported in filamentous fungi. These chromosomes vary from 0.39 to 0.77 Mb in size, and represent up to 38% of the chromosomal complement. Chromosome numbers among progeny isolates varied widely, with some progeny missing up to three chromosomes, while other strains were disomic for one or more chromosomes. Between 15–20% of the progeny isolates lacked one or more chromosomes that were present in both parents. The two high-density maps showed no recombination of dispensable chromosomes and hence, their meiotic processing may require distributive disjunction, a phenomenon that is rarely observed in fungi. The maps also enabled the identification of individual twin isolates from a single ascus that shared the same missing or doubled chromosomes indicating that the chromosomal polymorphisms were mitotically stable and originated from nondisjunction during the second division and, less frequently, during the first division of fungal meiosis. High genome plasticity could be among the strategies enabling this versatile pathogen to quickly overcome adverse biotic and abiotic conditions in wheat fields.     

New chromosome numbers for plant taxa endemic to the Balearic Islands

Mitotic chromosome numbers are reported from 25 vascular plant taxa, endemic to the Balearic Islands that are poorly known cytogenetically. The chromosome numbers ofAnthyllis vulneraria subsp.balearica (2n=12),Cymbalaria fragilis (2n=56), andPolygonum romanum subsp.balearicum (2n=40) were determined for the first time. A new chromosome number was found in several populations ofAnthyllis hystrix (2n=70) suggesting that this species is decaploid, in contrast to an earlier work reporting a higher ploidy level (2n=12x=84). The new chromosome number 2n=32 was reported inHypericum hircinum subsp.cambessedesii. It is suggested that the previous count (2n=40) could be explained by the presence of anomalous pentaploid cells in some tissues, contrating with the presence of a regular tetraploid complement (2n=32). Cytogenetic observations suggest thatSibthorpia africana has a diploid chromosome complement of 2n=18, with 0–2 accessory chromosomes. Accessory chromosomes are also reported forPhlomis italica, being the first record of B chromosomes in this genus. Chromosomal instability was found inGalium crespianum andG. friedichii species, with three numbers 2n=44, 55 and 66. Two cytotypes differing in ploidy level were documented within single plants. It is suggested that both species share a regular complement of 2n=44 and that the past hybridization events and formation of regenerating roots from the typical rootstock ofG. crespianum andG. friedrichii could be involved in the genesis of chromosome variants through partial endopolyploidy and concomitant somatic segregation.     

Gametogenesis and Reproduction of Seven Species of Pratylenchus

Three bisexual Pratylenchus species, P. penetrans, P. vulnus and P. coffeae have n = 5, 6 and 7 chromosomes, respectively, and reproduce by cross-fertilization. The monosexual P. scribneri comprises two chromosomal and reproductive forms. One has n = 6 chromosomes and reproduces by meiotic parthenogenesis, the other has a somatic chromosome number of approximately 25 and reproduces by mitotic parthenogenesis. The monosexual species P. zeae, P. brachyurus and P. neglectus have somatic chromosome numbers of approximately 21 to 26, 30 to 32, and 20, respectively, and reproduce by mitotic parthenogenesis. All mitotic parthenogenetic forms probably are polyploid. The phyletic relationships of some species are discussed briefly.     

Die Cytologie der Parthenogenese bei dem Tardigraden Hypsibius dujardini

Hypsibius dujardini Doy. (Articulata, Tardigrada) shows obligatory parthenogenesis under given cultivating conditions. Males were never found. The first meiotic division reduces the number of chromosomes; the (2n=10) chromosomes are divided between a small polar body and the egg nucleus. Prior to the second division the dyads divide, thus restoring the diploid number. A diploid polar body is formed subsequent to the second division. After the egg nucleus has moved toward the center of the egg, the cleavage divisions begin. — During meiosis II and the first cleavage divisions the chromosomes can develop into “large chromosomes” which presumably consist mostly of RNA. No “large chromosomes” are found after the seventh cleavage division. Sometimes a plate of coloured material (“elimination chromatin”) can be observed between the anaphase daughter plates of the first cleavage divisions. In this case the chromosomes are always small.     

Paternal Loss (PAL): A Meiotic Mutant in DROSOPHILA MELANOGASTER Causing Loss of Paternal Chromosomes

The effects of a male-specific meiotic mutant, paternal loss (pal), in D. melanogaster have been examined genetically. The results indicate the following. (1) When homozygous in males, pal can cause loss, but not nondisjunction, of any chromosome pair. The pal-induced chromosome loss produces exceptional progeny that apparently failed to receive one, or more, paternal chromosomes and, in addition, mosaic progeny during whose early mitotic divisions one or more paternal chromosomes were lost. (2) Only paternally derived chromosomes are lost. (3) Mitotic chromosome loss can occur in homozygous pal⁺ progeny of pal males. (4) Chromosomes differ in their susceptibility to pal-induced loss. The site responsible for the insensitivity vs. sensitivity of the X chromosome to pal mapped to the basal region of the X chromosome at, or near, the centromere. From these results, it is suggested that pal⁺ acts in male gonia to specify a product that is a component of, or interacts with, the centromeric region of chromosomes and is necessary for the normal segregation of paternal chromosomes. In the presence of pal, defective chromosomes are produced and these chromosomes tend to get lost during the early cleavage divisions of the zygote. (5) The loss of heterologous chromosome pairs is not independent; there are more cases of simultaneous loss of two chromosomes than expected from independence. Moreover, an examination of cases of simultaneous somatic loss of two heterologs reveals an asymmetry in the early mitotic divisions of the zygote such that when two heterologs are lost at a somatic cleavage division, almost invariably one daughter nucleus fails to get either, and the other daughter nucleus receives its normal chromosome complement. It is suggested that this asymmetry is not a property of pal but is rather a normal process that is being revealed by the mutant. (6) The somatic loss of chromosomes in the progeny of pal males allows the construction of fate maps of the blastoderm. Similar fate maps are obtained using data from gynandromorphs and from marked Y chromosome (nonsexually dimorphic) mosaics.     

The site of function of the Y chromosome in Drosophila melanogaster males

Summary The Y chromosome is essential for fertility in D. melanogaster males. An analysis of 126 pal-induced Y chromosome mosaics indicated that its function is only required in the germ line of fertile males. This analysis also showed that approximately 1/4 of all pal-induced Y chromosome mosaics had an XO/XYY constitution and hence that they resulted from somatic nondisjunction. Preliminary evidence suggests that pal-induced somatic nondisjunction can occur at the second or subsequent cleavage divisions.     

Oogenesis and Reproduction of the Birch Cyst Nematode,Heterodera betulae

Cytological study revealed that maturation of oocytes of Heterodera betulae is by regular meiosis and reproduction is by parthenogenesis. Restoration of the somatic chromosome number occurs after telophase II and before egg pronucleus formation, in the absence of a mitotic apparatus through a type of endomitotic division. The haploid chromosome number is 12 (2n = 24) in 95% of the female nematodes studied and 13 in the remaining 5%. The phylogenetic relationship of H. betulae with most other Heterodera species having n = 9 is not clear.     

Chromosomal and molecular rearrangements in somatic hybrids between tetraploid Medicago sativa and diploid Medicago falcata

The aim of this study was to produce somatic hybrids between tetraploid (2n=4x=32) M. sativa and diploid (2n=2x=16) M. ?falcata and analyse their genomic structure. Protoplasts from genotypes selected for regeneration ability from the cultivar Rangelander of M. sativa and Wisfal-1 of M. falcata were electrofused. Seven somatic hybrid calli were produced and one of them regenerated plants. The hybrid nature of these plants and their genetic composition were assessed with morphological, cytological, and molecular analyses. The resulting plants were hyper-aneuploid (2n=33) and contained one extra long chromosome, indicating that a translocation had taken place. The presence of both types of parental sequences in the RAPDs analysis confirmed the true hybrid nature of the plants. Rearrangements within the parental genomes and the presence of somaclonal variation among hybrid plants were observed through an RFLP analysis of the nucleolar organizing region (NOR). The possible causes for the gross genomic alterations, and the suitability of this method for transferring useful agronomic traits from wild species to cultivated alfalfa, are discussed.     

Evolutionary history of the mitochondrial genome in Mycosphaerella populations infecting bread wheat,durum wheat and wild grasses

Plant pathogens emerge in agro-ecosystems following different evolutionary mechanisms over different time scales. Previous analyses based on sequence variation at six nuclear loci indicated that Mycosphaerella graminicola diverged from an ancestral population adapted to wild grasses during the process of wheat domestication approximately 10,500 years ago. We tested this hypothesis by conducting coalescence analyses based on four mitochondrial loci using 143 isolates that included four closely related pathogen species originating from four continents. Pathogen isolates from bread and durum wheat were included to evaluate the emergence of specificity towards these hosts in M. graminicola. Although mitochondrial and nuclear genomes differed greatly in degree of genetic variability, their coalescence was remarkably congruent, supporting the proposed origin of M. graminicola through host tracking. The coalescence analysis was unable to trace M. graminicola host specificity through recent evolutionary time, indicating that the specificity towards durum or bread wheat emerged following the domestication of the pathogen on wheat.     

The evolution of the ribosomal loci in the subgenusLeopoldia of the genusMuscari (Hyacinthaceae)

In the subgenusLeopoldia of the genusMuscari, M. comosum is an exceptional species because it presents the most asymmetrical karyotype of the group and because its only active NOR is located in the fifth chromosome pair, while in the other species it is located in the first or second chromosome pairs (all the species have 2n = 18 chromosomes). SinceM. comosum has a derived karyotype different from those of the other species of the group, the resulting question is whether, in the first and second chromosome pair of this species, ribosomal cistrons persist. Observations after fluorescence in situ hybridization (FISH) using rDNA probes indicate that there are indeed ribosomal loci in the first and second chromosome pairs of this species, although these loci are inactive with respect to nucleolus organization. The location of rDNA regions in another three species of the same genus (M. atlanticum, M. dionysicum andM. matritensis) provides a basis for examining the significance of these findings in relation to the evolution of the ribosomal loci in this genus. Our observations indicate that in the genusMuscari, the largest sites for rRNA genes are not necessarily active, and, therefore, the activation of these regions is not related to the number of copies but to a specific regulation mechanism.     

Cytotaxonomical analysis of <Emphasis Type="Italic">Momordica</Emphasis> L. (Cucurbitaceae) species of Indian occurrence

Somatic chromosome number and detailed karyotype analysis were carried out in six Indian Momordica species viz. M. balsamina, M. charantia, M. cochinchinensis, M. dioica, M. sahyadrica and M. cymbalaria (syn. Luffa cymbalaria; a taxon of controversial taxonomic identity). The somatic chromosome number 2n = 22 was reconfirmed in monoecious species (M. balsamina and M. charantia). Out of four dioecious species, the chromosome number was reconfirmed in M. cochinchinensis (2n = 28), M. dioica (2n = 28) and M. subangulata subsp. renigera (2n = 56), while in M. sahyadrica (2n = 28) somatic chromosome number was reported for the first time. A new chromosome number of 2n = 18 was reported in M. cymbalaria against its previous reports of 2n = 16, 22. The karyotype analysis of all the species revealed significant numerical and structural variations of chromosomes. It was possible to distinguish chromosomes of M. cymbalaria from other Momordica species and also between monoecious and dioecious taxa of the genus. Morphology and crossability among the dioecious species was also studied. Evidence from morphology, crossability, pollen viability and chromosome synapsis suggests a segmental allopolyploid origin for M. subangulata subsp. renigera. The taxonomic status of the controversial taxon M. cymbalaria was also discussed using morphological, karyological and crossability data.